When the newsletter is being sent, only the heading of the post but not the rest of the content is being shown. I searched the forum but did not be able to get any information about that. (At least in English:)) Does anyone have a solution?

Well, looks like there's a bug in Next-Gen. You could try to find the Next-Gen filter in the filter list on the settings page an deactivate it. Pobably images woun't be included in mails, but at least it will work.